The Associated Press reported on Wednesday that the attacks on American diplomats in Cuba throughout the year may not be acoustic in nature, as doctors tell the outlet that they have found physical damage in patients’ brains.

Doctors say that an acoustic attack, as journalists had previously speculated occurred in Cuba, being responsible for such brain abnormalities would constitute an unprecedented medical occurrence.

The U.S. State Department has not discussed the details of the attacks on its staff, citing medical privacy issues, though Secretary of State Rex Tillerson has described the incidents in question as “attacks” and accused the Cuban government of having failed to prevent them. On Wednesday, Tillerson once again reiterated the belief of the Trump administration that the communist Castro regime could have stopped the harm, but failed to do so.

The Castro regime has denied any involvement in the attacks and the existence of the incidents entirely. A recent report on Cuban state television claimed that the disruptive sounds victims reportedly heard before suffering symptoms were crickets, cicadas, and other insects common to the tropical island.

AP reported Wednesday that the doctors studying the victims of the attacks—who complained of headaches, hearing loss, memory loss, and other concussion-like symptoms—had found “perceptible changes in their brains.” Specifically, the victims “developed changes to the white matter tracts that let different parts of the brain communicate,” something never before documented as the product of exposure to dangerous acoustic waves.

Martí Noticias notes that doctors have documented this type of damage in veterans of Iraq and Afghanistan who have survived explosions and not suffered any overt physical damage, but no reports indicate that the victims of the Cuba attacks experienced any explosions in their vicinity of blows to the head.

The outlet goes on to state that the University of Miami and University of Pennsylvania researchers studying the victims are expected to release findings in an article for the Journal of the American Medical Association (JAMA).

The report noted that some victims had fully recovered and had gone back to work, while others were still suffering the effects of the attack.

The incidents that led to the symptoms being recorded in Havana reportedly began occurring in November 2016, and the U.S. government does not have confidence that they have concluded. Some Canadian diplomats also reported similar symptoms to their American counterpart. Canada enjoys close relations with Cuba; its prime minister, Justin Trudeau, is a longtime family friend of the Castro family.

“The sounds may have been the byproduct of something else that caused damage, said three U.S. officials briefed on the investigation,” AP reported.

On Wednesday, at a press conference with NATO leaders, Tillerson insisted that American officials “are convinced that these were targeted attacks.”

“What we’ve said to the Cubans is: small island,” Tillerson told reporters, according to the New York Times. “You’ve got a sophisticated security apparatus. You probably know who is doing it. You can stop it. It’s as simple as that.”

“We understand the Cubans don’t like the actions we’ve taken. We don’t like our diplomats being targeted,” he added.

Tillerson also confirmed that American officials had shared information with the Cuban government in the hopes of aiding the investigation. The Cuban government has repeatedly complained that Washington had not included it sufficiently in the investigation.

Tillerson announced the withdrawal of all non-essential embassy staff from Cuba in October, as well as the reciprocal expulsion of 15 Cuban diplomats from their embassy in Washington. The Cuban government responded by calling the withdrawal “hasty,” accusing the United States of fabricating “science fiction” stories, and accusing the United States of not sharing the evidence necessary for Cuba to properly conduct an investigation into itself.

The communist government ultimately published a report on the findings of the investigation which absolved Cuba of any wrongdoing and accused the victims of the attacks of using insect sounds to claim an attack.